Ultrasonic transducers are used in many medical applications and, in particular, for the non-invasive acquisition of images of organs and conditions within a patient, typical examples being the ultrasound imaging of fetuses and the heart. The ultrasonic transducers used in such applications are generally hand held, and must meet stringent dimensional constraints in order to acquire the desired images. For example, it is frequently necessary that the transducer be able to obtain high resolution images of significant portions of a patient's chest cavity through the gap between two ribs when used for cardiac diagnostic purposes, thereby severely limiting the physical dimensions of the transducer.
As a consequence, and because of the relatively small aperture between human ribs and similar constraints upon transducer positioning when attempting to gain images of other parts of the human body, there has been significant development of linear or phased array transducers comprising multiple transmitting and receiving elements, with the associated electronics and switching circuits, to provide relatively narrowly focused and "steerable" transmitting and receiving "beams". The most common of such transducers comprises a one element wide by multiple element long linear array of transmitting and receiving elements arranged in line along a flat plane or, preferably, along a concave or convex arc, thereby providing a greater scanning arc.
The transmitting and receiving beams of such transducers are formed and steered by selecting individual transducers elements or groups of transducer elements to transmit or receive ultrasonic energy, wherein each such individual transducer element or group of transducers elements forms an "aperture" of the transducer array. Such an array is thereby formed of a single row of apertures extending along the face of the array and such transducers are consequently referred to as "single aperture" transducers.
While such azimuth scanning single aperture arrays are advantageous for many applications, single aperture transducers have the disadvantage that they can scan only along the single plane of the transducer elements. As a consequence, there have been many attempts to construct transducers that are also capable of steering or focusing in elevation as well as azimuth, that is, along the axis at right angles to the azimuth plane along which the elements are arrayed as well as along the azimuth plane.
As is well understood, the formation and steering and/or focusing of the transmitting and receiving beams of a transducer are controlled by selection and use of the various separate physical divisions or areas of transducer material comprising the transducer array, which, as described above, are referred to as "apertures". In contrast to "single aperture" transducers, however, in which each aperture is formed by an element or group of elements extending across the face of the array as a single unitary area or division or the array, each corresponding element in a transducer capable of scanning in elevation is divided into multiple sub-elements, or segments. For this reason, and because each element position along such an array can form multiple apertures, that is, using different combinations of the sub-elements or segments of each of the transducer elements, such transducers are consequently referred to as "multiple aperture" transducers.
The shape, focus and direction of the transmitting and receiving beams of a multiple aperture transducer are again controlled by selection of the apertures of the array. In a multiple aperture array, however, each aperture is formed by one or more of the sub-elements, or segments, of the transducer elements, so that the apertures of a multiple aperture array can be used to steer and focus the transducer scan beam along the elevation axis as well as along the azimuth axis and can define multiple azimuthal scan planes, each being at a different angle of elevation.
It should be noted that in both single aperture transducers and in multiple aperture transducers the apertures may be either driven actively, or simply deactivated to reduce the size of the acoustic aperture, thereby controlling the shape, direction and focus of the transmitting and receiving beams formed by the transducer array.
The transducer elements of both single aperture and multiple aperture transducers are generally made of a piezoelectric material and the array of elements or sub-elements is generally mounted onto a body made of a backing material. Connections between the individual transducer elements and the associated electronics and switching elements are usually provided through various arrangements and combinations of thick and thin film circuits, flexible printed circuits and wires, which are generally located on the back of the array, between the array and the body, with leads running along the body to the transducer electronics. One or more layers of impedance matching material, generally considered to be a part of the elements themselves, is often superimposed upon the transducer elements to match the acoustic impedance of the transducer to the body or material being scanned, and a lens comprised of a suitable material may be additionally superimposed upon the impedance matching material to shape or focus the beams generated by the transducer elements. In some implementations, the impedance matching layers may have suitable acoustic characteristics and may be shaped to operate as an acoustic lens.
Single aperture transducers are generally constructed from a single piece of transducer material having a width equal to the length of one element and a length equal to the widths of the total number of elements plus spaces between the elements. One or more thin or thick film circuits or flexible printed circuits having connections and paths for the individual elements, or the like implemented in any of several other ways, are bonded to one side of the piece of transducer material and a layer or layers of matching material may be bonded to the radiating and receiving side of the transducer material to form a "stack" of the transducer material, circuits and matching layers. A temporary or permanent layer of backing material of some form, such as a flexible material, may also be bonded to the back of the stack to aid in handling the stack during manufacture.
Successive cuts are then made across the width of the transducer stack on the radiating/receiving side of the stack and at intervals corresponding to the widths of the elements and the spacing between the elements to divide the single piece of material into the individual elements. This operation is generally referred to as "dicing" and is usually done with a device referred to as a dicing saw, but may be done with other techniques, such as lasers. These cuts may extend only through the transducer and matching material layers, or partly or completely through the circuit layer, or through the circuit layer and at least a part of any backing layers, depending upon the detailed design and implementation of the circuit layers.
The assembly of individual transducer elements with the circuit and matching layers are then bonded to the backing body, which may have a flat, concave or convex face, as described above, with any temporary backing layers being removed as necessary. It should be noted that in certain instances the dicing may be done after the assembly of transducer elements, matching materials, and circuits is bonded to the backing material and that the dicing cuts may extend into the layers of backing material or even into the backing body.
Connections between the thin or thick film circuits connecting to the transducer elements and wires or printed circuits, such as flexible circuits, which in turn connect to the electronics and switching elements may made before or after the transducer assembly is bonded to the backing body, again depending on the detailed design and implementation of these connections.
While methods for the construction of multiple aperture transducer are well known, and similar to those used in construction of single aperture arrays, multiple aperture arrays present greater difficulties than do single aperture arrays. For example, a particular application may require that each element be comprised of three segments, or apertures, that is, two outer segments and a middle segment. This may be achieved, for example, by constructing the transducer elements from three elongated pieces of transducer material, that is, two outer pieces and a middle piece, and then dicing the pieces across the face of the array as was described with regard to single aperture arrays, or by additional cuts along the transducer stack in the longitudinal direction to divide the two outer segments from the middle segment.
A primary problem in constructing transducers, however, is in achieving the electrical connections to the elements and sub-elements, or segments, as the number of elements or sub-element segments increases. That is, the physical dimensions of an array, especially for medical use, is generally constrained, for example, by the need to scan the cardiac structures through the space between patient's ribs to avoid interference by the ribs. At the same time, there is a need and trend to increase the number of elements or sub-elements to achieve every finer scan resolution to achieve increasingly detailed images of the cardiac structures.
While this problem exists even with single aperture transducers, the problem is particularly severe with multiple aperture transducers because the number of electrical connections to each element, each of which may be comprised of three or more segments, or sub-elements, is greatly increased while the space in which to make the connections does not increase. For example, in a single aperture array each element is made of a single segment while in a three aperture array each element is divided into three segments. As a result, while each element of a single aperture array requires a single connection to the single segment that comprises the element, a three aperture array requires, for each element, two separate connections to the two outer segments and a third connection to the middle segment, thereby tripling the number of connections per element, and possibly requiring additional connections to each possible pair of segments. In addition, each middle segment is bounded on both ends by the outer segments of the element and on either side by the two adjacent elements, so that the middle segments are not readily accessible for connections. It is therefore apparent that the space available to make connections to the segments of a multiple aperture array and to run the leads from the segments to the points of connection to the transducer electronics is extremely constrained and that the problem compounds very rapidly as the number of elements in the transducer or the number of segments in each element increases.
Considering a specific example, the Hewlett-Packard Model 21215 transducer provides two sizes of elevation apertures and is constructed generally as described above, that is, of a linear array of separate or separated elements wherein each element is comprised of three separate segments, two outer segments and a middle segment. In this design, the elements are arranged in a straight plane, rather than a concave or convex arc, and the middle segment of each element is connected to a transmit/receive circuit while the two outer segments of the element are connected together and then to a second transmit/receive circuit or through a switch to the same transmit/receive circuit as the middle segment.
Connections to the segments are made through flex circuits, that is, circuits etched onto thin, flexible circuit boards, wherein an individual flex circuit is used for each set of elevation segment connections and wherein each flex circuit contains all of the connections for the corresponding segments of each of the elements along the array. The transducer therefore requires three flex circuits, one for each out row of segments and one for the middle row of segments. The two flex circuits connecting to the outer segments of each element of the outer segments and are then connected by a flex circuit having jumper connections, or by a circuit board. The third flex circuit connects to the middle segments of the elements, and thus must make connection at the middle of the back side of the piezoelectric array.
It is therefore apparent that a three aperture array like the Hewlett-Packard Model 21215 requires three times as many connections to the piezoelectric segments themselves and twice as many flex circuits as in a single aperture array, and two additional flex circuit to flex circuit connections through flex jumper connections or through a printed circuit board for each element. These connections result in higher cost and lower manufacturing yield. In addition, assembly is more complex in that the flex circuit to the middle segments must be carefully aligned with the flex circuits to the outer segments. This factor alone makes it difficult, if not impossible, to manufacture a curved array and the presence of the middle segment flex circuit requires the use of either a poured backing body material or complex molding or machining to manufacture the backing body.
To further compound the problem of achieving a large number of connections and leads to the transducer elements and segments in a small area, the connections to the segments must be made in such a manner as not to interfere with the acoustic characteristics of the transducer. That is, it has been described above that the connections to the transducer elements and segments are generally made through thin or thick film circuits or flex circuits bonded to the back side of the transducer elements. The number of leads and connections, however, generally results in a connection and lead layer or layers having significant thickness and effect, in terms of the acoustic characteristics of the array, thereby distorting or interfering with the acoustic characteristics of the array. In addition, the lead and connection layer or layers and other layers interposed between, such as insulating layers, do not provide smooth surfaces, or planes, because of the raised or depressed areas of the layers forming the leads and connections. As such, it is difficult to reliably bond the layers together without significant additional layers of bonding materials and the unevenness of the surfaces tend to trap bonding material and air between the layers, thereby providing an acoustically non-homogenous "body" bonded to the "back" face of the transducer elements that further interferes with the acoustic characteristics of the transducer array.
The methods used in the prior art to construct multiple aperture arrays include the use of multiple flex circuits, as described just above, connections embedded in the backing body, the use very thin film or deposited circuits to form the connections and leads to the transducer elements and segments, and even the use of electrostrictive rather than piezoelectric materials for the transducer elements.
Each of these methods, however, provides its own difficulties and problems. For example, the disadvantages of multiple flex circuits have been discussed above, and the disadvantages of connections embedded in the backing body are comparable.
An alternative is the use of a multi-layer thick film ceramic hybrid circuit which also serves as the backing body. The laminated layers with embedded connection circuits results in leads which run vertically, that is, perpendicularly, between the segments and an interface circuit to which the connections are made, but also results in leads with very small cross sections that are attached at both ends by butt joints, which lack reliability. The use of a multi-layer thick film circuit, in turn, can provide much stronger and more reliable connections, but the acoustic characteristics of the ceramic material may degrade the acoustic performance of the transducer. Both approaches, moreover, may have the disadvantage of requiring multiple steps to make the connections to the piezoelectric elements and may result in added cost from not using standard printed or hybrid circuit manufacturing techniques.
Yet other approaches use thin film or very thin film circuits for the connections and leads, thereby providing connection and lead layers that are acoustically thin and thereby cause less interference with the acoustic characteristics of the transducer. Thin film circuits, however, are difficult to work with in manufacture, often being relatively fragile, and generally require "wet" manufacturing processes that result in potentially undesirable materials to be disposed of.
In addition, thin film circuits, like thick film circuits and flexible circuits, require connections between layers, for example, between the layer forming contacts to the elements and segments and the layer providing the interconnecting leads, and these interlayer connections, commonly called "vias" are difficult to form in the thicknesses typical of thin film circuits. Certain of the prior art approaches to thin film circuits, for example, while recognizing the advantages of thin film circuits for the actual contacts to the transducer elements and segments and for the interconnecting leads, have required the use of additional, vertically oriented circuit boards or very thin, free standing wires to accomplish the necessary connections.
The present invention provides a solution to these and other problems of the prior art.